In a magnetic disk drive, the following motions (load/unload) are defined. Specifically, in order to prevent a contact of a head with a disk surface, the head is removed from the disk surface onto a ramp mechanism if an impact is given to a magnetic disk drive and at the time of power-off or idling. This motion is hereinafter referred to as an unload. Further, when the head is moved from the ramp mechanism to the disk surface, this motion is hereinafter referred to as a load.
Further, in the magnetic disk drive, a head positioning control system controls a head based on a head position error signal (PES signal) obtained by reproducing data included in a servo sector. However, it is impossible to obtain a head position error signal when a head is positioned on a ramp mechanism without being positioned on a disk surface in the foregoing load/unload motions. For this reason, generally, the following control is carried out. Specifically, a head velocity is estimated from a back electromotive voltage generated in a voice coil motor (VCM) for moving a head. Therefore, the velocity control of the head is carried out based on the foregoing estimated value.
When a head velocity is estimated from a back electromotive voltage, a drop in voltage caused by a coil resistance is subtracted from a detected terminal voltage of the coil shown in the following equation (1).Back electromotive voltage=terminal voltage of coil−coil resistance×coil current  (1)
In the foregoing equation (1), the coil resistance is a previously known constant. However, if a magnetic disk drive is used for a long time, a temperature change occurs due to the influence of a coil current; as a result, a coil resistance changes. Moreover, there is a high probability of resistances varying due to a change in individual coil resistances.
Therefore, there is a need to obtain an accurate coil resistance before load/unload is carried out. Patent document 1 (Japanese Patent No. 4,180,582) discloses an apparatus for estimating a coil resistance in a load motion. In the foregoing apparatus, a current is carried so that a head gimbal assembly (HGA) is urged against the ramp mechanism dead-end direction, and then, a coil resistance is estimated in a state that a back electromotive voltage is set to 0. Further, Patent document 2 (Jpn. Pat. Appln. KOKAI Publication No. 11-25626) discloses the following method. According to the foregoing method, in an unload motion, a head gimbal assembly (HGA) is urged against a stopper on the disk inner circumferential side. However, the probability of a contact of a head with a disk surface is high; for this reason, the foregoing method is not preferable. Therefore, there is a need to estimate a coil resistance before an unload motion using a method similar to a normal seek operation.
Furthermore, Patent document 3 (Jpn. Pat. Appln. KOKAI Publication No. 2001-344918 (paragraph [0013])) discloses the following apparatus. According to the foregoing document 3, a back electromotive voltage Vbemf has a proportional relationship to a velocity estimated value (i.e., back electromotive voltage Vbemf=K (proportional gain)×velocity estimated value). The foregoing proportional gain K is already known; therefore, the velocity estimated value is obtained, and thereby, a back electromotive voltage Vbemf is inevitably obtained. However, if individual variations and changes occur in proportional gain K, there is a possibility that inaccurate estimation is performed.
According to a conventional case, it is impossible to estimate a coil resistance using a method similar to a normal seek operation.